The field of art to which this invention pertains is aryl cyanate esters, i.e., cyanic acid esters of polyhydric phenols.
Cyanate esters are finding increasing use in structural composites, adhesives, and electrical grade insulation. These esters are based on the reaction products of polyhydric phenols and cyanogen halides. Cyanate esters and processes for preparing them are described in a number of patents, examples of which are U.S. Pat. Nos. 3,403,128; 3,755,042; 3,987,230; 4,028,393; 4,330,658; and 4,839,442.
Industry is constantly searching for materials which can be readily processed into products having improved performance properties. There is a need for thermosetting resin compositions which have viscosities of less than 20,000 cps at 25.degree. C. or have crystalline melting points below 100.degree. C. There is also a need for low Tg resin compositions which will cure at temperatures as low as 120.degree. C. to give compositions having low water absorption properties as well as low dielectric loss properties.
Ovens large enough to be used to cure the thermosetting matrices of large composite structures, such as aircraft wings and fuselages, submarine hulls, and antenna reflectors, are not available or practical. Such large structures are generally placed under a tent-like cover and are heated with hot air. The maximum practical temperature which can be obtained under such conditions will range from about 250.degree. F. to about 300.degree. F. (121.degree. C. 149.degree. C.). There is a need for resin systems which will cure in this temperature range.
Composite structures will undergo less outgassing during deployment in the vacuum environment of outer space when the matrix resins absorb less moisture during processing in earth's atmosphere. Microcracking due to thermal cycling imposed when the orbiting space structure passes through the earth's shadow can be reduced or eliminated by incorporating strain-at-break-enhancing thermoplastics into liquid thermosetting resins and/or by reducing residual curing stresses.
Multilayer circuit boards which utilize aramid fibers to match low coefficient of thermal expansion of surface mounted chip packages can similarly benefit in terms of microcrack resistance by employing resins which develop less cure stress or tolerate higher strains.
Absorption of microwave energy by radomes decreases with low dielectric constant (Dk) composite materials. Power loss in antennas is reduced with lower dissipation factor (Df) materials.
Signal propagation delays are reduced in high frequency circuit boards by reducing the Dk of substrate materials. Interconnect materials require a flat Dk response well into the gigahertz frequency range to avoid echo delays caused by mismatched impedance values. Power loss is reduced with decreasing Df values in substrate materials.